I often have to argue or even fight with storage providers about cache sizing. Here is a set of rules I apply in Proof Of Concepts and disk I/O modellization.
Write cache size:
1. Cache sizing : the real issue is not the cache size, but how fast the cache can flush to disk. In other words, assuming sustained IO, cache will fill and I/O will bottleneck is this condition is met : the rate of incoming I/O is greater than the cache ability to flush.
2. The size of a write cache matters when the array must handle a burst of write I/O. A larger cache can absorb bigger bursts of write, such as database checkpoints. The burst can then be contained without hitting forced flush.
3. Write caching from one SP to the other is normally activated for redundancy and single point of failure removal. That is, the write cache in each SP contains both the primary cached data for the logical units it owns as well as a secondary copy of cache data for the LUNs owned by its peer SP. In other words, SP1’s write cache hold a copy of SP2’s write cache and vice versa. Overall, the real write cache size (seen from a performance point of view) is half the write cache configuration.
4. Write caching is used for raid 5 full stripe agregation (when the storage firmware support this feature) and parity calculation, a very useful feature for many large environments. Lack of space must not force the cache to destage.
Read cache size:
1. Randoms reads are little chance to be in cache: the storage array read cache is unlikely to provide much value on top of the SGA and possibly the file system buffer cache (depending of FILESYSTEMIO_OPTIONS and file system direct IO mount options).
2. However, array read caches are very useful for prefetches. I can see three cases for this situation to occur :
- Oracle direct reads (or direct path reads) are operating system asynchronous reads (I’ll write a new post on this later). Assuming a well designed backend, you will be able to use a large disk bandwidth to take advantage of large numbers of parallel disk I/O.
- Buffered reads (imports for example) will take advantage both of file system and storage array read ahead operations.
Also take a look at http://www.oracle.com/technology/deploy/availability/pdf/oow2000_same.pdf